1. Field of the Invention
The present invention relates to a crank angle sensor signal processor, and more specifically to a crank angle sensor signal processor for an engine, which is resistant against noise.
2. Description of the Prior Art
Recently, an engine is usually controlled by a microcomputer, and various sensors for detecting engine operating conditions are connected to the microcomputer. A crank angle sensor is one of the important sensors connected to a microcomputer control system. The crank angle sensor generates a reference crank angle signal (e.g. 120.degree. signal in the case of a six-cylinder engine) and a unit crank angle signal (e.g. 1.degree. signal) in order to control ignition timings of an engine, for instance, as is well known.
In controlling ignition timings of an engine, an actuator (e.g. switching power transistor) is turned off, for instance, in response to an ignition timing signal generated from a microcomputer to cut off the primary current of an ignition coil, so that the engine is sparked. In practice, the switching power transistor is turned off for ignition when the crank angle reaches a predetermined angular position (i.e. at ignition timing) previously stored in a memory unit of the microcomputer. The crank angular positions can be determined on the basis of the 120.degree. signal and the 1.degree. signal in the case of a six-cylinder engine.
An assumption is made that ignition timing data under some engine operating conditions is 40 degrees in advance of the top dead center. In usual, the 120.degree. signal is generated at a crank angular position 70 degrees in advance of the top dead center. In response to this 120.degree. signal, control begins to count the number of the 1.degree. signals. Therefore, after the 1.degree. signals have been counted 30 times and immediately after the 31st 1.degree. signal has been inputted, the switching power transistor is turned off to ignite the engine cylinder at a crank angular position 40 degrees in advance of the top dead center.
In practice, however, the reference crank angle signal (REF signal) indicative of a reference crank angular position for ignition timing control is generated at an angular position 4 degrees in retard of the 120.degree. signal (i.e. 66 degrees in advance of the top dead center, as shown in FIG. 1A.
In response to this reference crank angle signal (REF SIG), control generates an interrupt demand signal to execute an ignition. In the following description, however, the 120.degree. signal is assumed to be the REF signal without distinguishing these two signals for simplification.
In the prior-art crank angle sensor signal processor, however, there exists a problem in that high frequency noise is easily superposed upon the REF signal (180.degree. signal in the case of 4-cylinders; 120.degree. signal in the case of 6-cylinders) through the sensor wire. When the noise level exceeds a predetermined level, an interrupt signal generator erroneously recognizes noise as the REF signal and therefore generates an interrupt demand signal to the controller (CPU of the microcomputer) for ignition.
As a result, the switching power transistor is turned off erroneously to generate an ignition pulse in spite of non-ignition timing, thus deteriorating engine performance momentarily.
To overcome the above-mentioned problem, it may be possible to incorporate noise eliminating hardware in an interface between the sensor and the microcomputer. However, the additional hardware will increase the cost of the crank angle sensor signal processor.